This invention relates to a circuit for automatically adjusting the frequency and the phase of an oscillatory circuit electrical signal useful for energizing an electroacoustic converter and, more specifically, concerns an electrical circuit for maintaining an oscillatory circuit of an ultrasonic apparatus tuned for optimum operation.
Ultrasonic energy has found wide use in industry for cleaning, welding, soldering, treating materials, homogenizing, dispersing, disrupting microbiological cells and the like. The sonic or ultrasonic energy necessary to accomplish such tasks is generated most often by means of an electroacoustic converter which converts high frequency electrical energy into mechanical vibration. When the mechanical or acoustic energy so produced is concentrated and increased in amplitude, by means of a suitably dimensioned horn to provide high power densities per unit area, such power is eminently suited for the ultrasonic processes described above. A typical electroacoustic converter is described in U.S. Pat. No. 3,524,085, issued to A. Shoh, entitled "Sonic Transducer" and dated Aug. 11, 1970. For a discussion on dimensioning horns see, for instance, the book entitled "Ultrasonic Engineering" by Julian R. Frederick, John Wiley & Sons, Inc., New York, N.Y., 1965, pp. 87 to 103.
One of the basic requirements for operating a high power sonic converter is that the converter and horn assembly be dimensioned to be mechanically resonant at a predetermined frequency usually in the range between about 1 kHz and 100 kHz and usually at a frequency of 20 kHz or 40 kHz. Maximum energy transfer from the electrical oscillator (power supply) to the mechanical system (converter and horn) occurs at the resonant frequency. The mechanical system may be resonant at frequencies close to but different from the predetermined resonant frequency. When the electrical and mechanical systems operate at frequencies other than the predetermined frequency, component failure in the power supply is a likely occurrence and the efficiency of the energy transferred from the power supply to the mechanical system is reduced. Overload circuits are generally provided to turn the power supply off in the event of operation at a frequency which differs from the predetermined frequency. A typical overload circuit is described in U.S. Pat. No. 3,946,280, issued to E. Quist, entitled "Overload Protection Circuit", dated Mar. 23, 1976.
In prior art arrangements, the power supply is mechanically tuned by providing an operator adjustment in conjunction with the viewing of a power meter. The tuning adjustment, typically, involves the positioning of a movable core within an inductor to thereby change the circuit inductance, and hence the resonant frequency of the power supply. Such an arrangement is currently found in commercially available ultrasonic welders, such as the Model 800 ultrasonic welder manufactured by Branson Sonic Power Company, Danbury, Conn. While this technique is very successful, mechanical retuning is required each time the electroacoustic converter or horn is replaced.
The present invention obviates the requirement for mechanically tuning the system by providing an automatic tuning circuit which maintains the oscillating frequency and phase of the power supply to correspond to the predetermined frequency of the converter and horn assembly. The phase of two electrical signals generated within the power supply are compared with one another. In the preferred embodiment, the first signal compared is the collector voltage signal and the second signal is the base voltage signal of one of the transistors of the oscillatory circuit comprising a portion of the power supply. A control signal is generated having an amplitude commensurate with the magnitude of the phase difference between the two signals and a polarity commensurate with which one of the two signals leads the other. The control signal is processed for providing an adjustment signal which is used to automatically vary the inductance of an electrically variable inductor in the feedback path of the oscillatory circuit. The use of an electrically variable inductor in lieu of the heretofore employed mechanically adjustable inductor in the feedback loop of a power supply and the generation of an adjustment signal for varying the inductance comprises the present invention.
A principal object of the present invention is, therefore, the provision of an automatic tuning circuit in an oscillatory circuit used for driving an electroacoustic converter, the tuning circuit adjusting the phase of a feedback signal from the output transformer energizing the converter to the input transformer of the oscillatory circuit.
Another object of the present invention is the provision of an automatic tuning circuit for maximizing energy transferred from a power supply to an electroacoustic converter at a predetermined frequency.
Further and still other objects of the present invention will become more clearly apparent from the following description when read in conjunction with the accompanying drawings.